This invention relates to an arrangement of a valve assembly and in particular although not exclusively a bleed valve assembly for a compressor stage of a gas turbine engine.
In a multi-stage compressor of a gas turbine engine each compressor stage is designed so that it operates as efficiently as possible over as wide a range of operating conditions as possible. However, for a gas turbine engine of an aircraft there are some operational conditions that depart from the capability of the compressor stage and there can be a breakdown of air flow and/or aerodynamically induced vibration occurs. If the engine demands a pressure rise from a compressor which is higher than can be sustained then xe2x80x9csurgexe2x80x9d occurs. Surge is an instantaneous breakdown in flow through the engine and the high pressure air in the combustion system is expelled forward through the compressor with a resultant loss of engine thrust. Compressors are therefore designed with an adequate xe2x80x9csurge marginxe2x80x9d to ensure that this instability is avoided. However, too great a surge margin leads to an inefficient engine at constant operating conditions such as cruise, thus the surge margin is kept to a minimum. To help reduce this surge margin variable inlet guide vanes and variable stator vanes are used to provide an acceptable air angle on to the compressor blades. Furthermore bleed valves are used between compressor stages to further and quickly reduce the pressure therein by venting compressed air, from a compressor stage of the engine, and thereby restoring the surge margin and operability of the compressor stage.
The present invention is concerned with the design of bleed valves. Currently one type of bleed valve in use is a pressure balanced valve where the pressures both sides of a valve face are equilibrated by passage of compressed air through holes in the valve face. However, this current design does not accommodate the pressure differences sufficiently and quickly enough and unpredictable performance in some circumstances include shutting of the valves under conditions when they should remain open and the valves remaining partially open when commanded to close.
It is therefore an object of the present invention to provide bleed valve apparatus which obviates the above problem.
According to the present invention there is provided a bleed valve for regulating a fluid flow through a bleed hole, the bleed hole is defined by a casing of a gas turbine engine compressor, the bleed valve comprises a central axis, a piston and a static structure, the static structure generally surrounding the piston, and is arranged to define in axial sequence from the bleed hole first, second and third chambers, the piston comprises a spindle, walls, a first end plate slidably sealed against the static structure and a valve face end plate from which the walls axially extend, the walls being slidably sealed to the static structure, the static structure comprises a radially inwardly extending flange, the flange defining an aperture through which the spindle axially extends and is slidably sealed against, the first chamber is in fluid communication with the compressor via pressure balancing apertures defined in the valve face end plate, the third chamber is also in fluid communication with the compressor via a passageway defined by the spindle, the bleed valve further comprises means for moving the piston between an open position and a closed position, wherein the valve face end plate comprises radially extending pressure balancing apertures.
Preferably, the means for moving the piston comprises a means for supplying compressed air into and venting compressed air from the second chamber, such that supplying compressed air moves the piston between the open position and the closed position, and a resilient member disposed to and between the flange and the first end plate and which is arranged to provide a returning force to the piston to move the piston between the closed position and the open position.
Preferably, the radially extending pressure balancing apertures substantially extend from the spindle to the axial walls and comprise a constant aperture width.
Alternatively, the radially extending pressure balancing apertures comprise an aperture width which tapers radially inwardly. Alternatively, the radially extending pressure balancing apertures comprise an aperture width which tapers radially outwardly.
Preferably, the radially extending pressure balancing apertures are symmetrically disposed around the valve face end plate.
Preferably, the casing is a casing surrounding a compressor stage of a gas turbine engine. Furthermore it is preferable for a gas turbine engine to comprise a bleed valve in accordance with the present invention.
Preferably, a method of operating a gas turbine engine comprising a bleed valve in accordance with the present invention comprises the steps of: when the engine is not operating the valve is held in the open position by the force exerted by the resilient means; on engine start up compressor air enters and pressurises the third chamber sufficiently to overcome the force exerted by the resilient means and forces the piston to move between the open position and the closed position, thereby preventing compressor air passing through the bleed hole, during this step air is vented from the second chamber via the means for supplying compressed air into and venting compressed air from the second chamber; at a first normal operating condition of the engine the bleed valve is closed; during a transient engine operating stage, between the first and a second normal operating condition, and when the pressure in the compressor reaches a predetermined level the means for supplying compressed air is commanded to permit compressed air into the second chamber, at a sufficient pressure, to move the piston between the closed position and the open position, thereby allowing compressor air to be exhausted through the bleed hole and reducing the pressure in the compressor; during the transient engine operating stage and when the pressure in the compressor drops below the predetermined level the means for supplying compressed air is commanded to permit venting of compressed air from the second chamber, in so doing the pressure in the third chamber is sufficient to move the piston between the open position and the closed position, thereby preventing compressor air exhausting through the bleed hole.